Investigation of nonlinear absorption processes with femtosecond light pulses in lithium niobate crystals

The propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNbO3) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is Ipa3.5cmaGW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776nm. The dependence of the probe transmission coefficient on the time delay It between probe and pump pulses is characterized by a narrow dip (at Ita0) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is Ira0.9cmaGW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776nm is Ïra8 -10a18cm2. The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of 0.24ps. © 2005 The American Physical Society.